What should the worshiper do if he catches the last رکعah of the Dhuhr prayer with the Imam, and is the following رکعah considered the second رکعah, with the Tashahhud recited twice, making the total number of Tashahhud recitations three times?

The rak'ah performed with the Imam is the beginning of the prayer, and the rak'ahs that follow after leaving the Imam are the end of the prayer. Sitting in the first rak'ah is necessary to follow the Imam, and in the second, it is one of the obligations of the prayer. As for the third and final sitting, it is one of the fard (obligatory) acts of prayer. Therefore, whoever catches the last rak'ah of a four-rak'ah prayer sits and recites the tashahhud three times.